The present invention relates to magneto-optical recording media, such as magneto-optical disks, magneto-optical tapes and magneto-optical cards, and a reproducing device for reproducing information on such a magneto-optical recording medium.
A magneto-optical recording medium has been practically used as a rewritable optical recording medium. Information is recorded on and reproduced from a magneto-optical recording medium with a converged light beam emitted from a semiconductor laser. However, the magneto-optical recording medium has such a drawback that the reproduction properties deteriorate when the diameter of a recording bit as a recording-use domain and the interval of the recording bits become smaller with respect to the diameter of the light beam.
When the diameter of the recording bit and the interval of the recording bits become smaller with respect to the diameter of the light beam, a recording bit adjacent to a target recording bit enters into the diameter of the light beam converged on the target recording bit. As a result, individual recording bits can not be read out separately and the reproduction properties deteriorate.
A structure for solving the above drawback of the magneto-optical recording medium is proposed in xe2x80x9cHigh-Density Magneto-Optical Recording with Domain Wall Displacement Detectionxe2x80x9d (Joint Magneto-Optical Recording International Symposium/International Symposium on Optical Memory 1997 Technical Digest, Tu-E-04, p. 38,39). In this magneto-optical recording medium, the first, second and third magnetic layers are layered in this order. The first magnetic layer is made of a perpendicularly magnetized film having a relatively small wall coercivity and a relatively large wall mobility compared with those of the third magnetic layer in the vicinity of a readout temperature. The Curie temperature of the second magnetic layer is set lower than the Curie temperatures of the first and third magnetic layers. According to this structure, even when the recording bit diameter and the recording bit interval are small, individual recording bits can be read out separately without lowering the readout signal level, by moving the domain wall into a region where the temperature has risen by the irradiation of a light beam.
A method of reproducing information on the magneto-optical recording medium with the above-described structure will be explained with reference to FIG. 10. A first magnetic layer 101, a second magnetic layer 102 and a third magnetic layer 103 are layered in an exchange coupled state. Denoting the Curie temperatures of the first, second and third magnetic layers in the laminated state by Tc101, Tc102 and Tc103, respectively, Tc101 and Tc102 satisfy the relationship Tc102 less than Tc101. In FIG. 10, the arrows show the direction of transition metal magnetic moments of the respective magnetic layers. Here, magnetic domains have already been recorded in the third magnetic layer 103, and an upwardly oriented magnetic domain and a downwardly oriented magnetic domain are present alternately in a repeated manner.
When a reproduction-use light beam 106 is irradiated and converged on such a magneto-optical recording medium from the first magnetic layer 101 side, the second magnetic layer 102 has a region heated to a temperature equal to or higher than its Curie temperature. In a region having a temperature lower than the Curie temperature, the magnetic domain information in the third magnetic layer 103 is copied to the first magnetic layer 101 through the second magnetic layer 102 by the exchange coupling. In other words, the upward transition metal magnetic moment at the front part of a region 110 irradiated with the light beam is copied as it is from the third magnetic layer 103 to the first magnetic layer 101.
On the other hand, in the region heated to a temperature equal to or higher than the Curie temperature of the second magnetic layer 102 (the region located behind a light beam 106 by a movement of the medium such as a rotation of a disk substrate), since the exchange coupling between the first magnetic layer 101 and third magnetic layer 103 is cut off by the second magnetic layer 102, the domain wall in the first magnetic layer 101 is readily movable.
When the information in the third magnetic layer 103 is copied as it is to the first magnetic layer 101, a domain wall 107 is essentially formed. However, in a region where the second magnetic layer 102 has been heated to a temperature equal to or higher than its Curie temperature, since the domain wall in the first magnetic layer 101 is readily movable, the domain wall 107 moves to the most stable location. Here, considering a fact that the domain wall energy density decreases with an increase in temperature, the domain wall 107 moves to a location where the temperature is increased most by the irradiation of the light beam 106, and forms a domain wall 108.
Thus, in the magneto-optical recording medium of the above-described structure, since the domain wall can be moved by the characteristic of the second magnetic layer 102, the recording domain of the third magnetic layer 103 can be enlarged in the first magnetic layer 101. Therefore, even when the recording domain is reduced, it is possible to increase the amplitude of the readout signal from the first magnetic layer 101, thereby allowing readout of signals of a cycle less than the diffraction limit of light.
However, in the above-mentioned reproduction method, there are two types of domain movements, i.e., a domain movement from the front part and a domain movement from the rear part. Hence, there is a problem that a single domain is read out twice. Referring now to FIGS. 11 and 12, the following description will explain this point.
FIG. 11 shows a state in which an independent magnetic domain 109 formed in the third magnetic layer 103 is present at the front part of the light beam 106, the third magnetic layer 103 and first magnetic layer 101 are exchange coupled at the position of the independent magnetic domain 109, and the upward moment is copied to the first magnetic layer 101. In FIG. 11, the shaded portion of the second magnetic layer 102 is a region X where the second magnetic layer 102 is heated to its Curie temperature or a higher temperature.
In the state shown in FIG. 11, the domain wall 107 moves to the position of the domain wall 108 to enlarge the magnetic domain, and a readout magnetic domain 111 with an upward moment is formed in the region 110 irradiated with the light beam 106. Therefore, a large readout signal amplitude is obtained. When the medium (magneto-optical recording medium) is moved relatively to the light beam 106 from the state shown in FIG. 11, a downward moment of the third magnetic layer 103 is copied to the first magnetic layer 101 upon passage of the independent magnetic domain 109 through the region X, and the moment in the readout magnetic domain 111 is also oriented downward.
Further, when the medium is moved into a state shown in FIG. 12, i.e., the independent magnetic domain 109 is located at the rear end of the region X of the second magnetic layer 102, the upward moment of the independent magnetic domain 109 in the third magnetic layer 103 is copied to the first magnetic layer 101, and a domain wall 107xe2x80x2 moves to the position of a most stable domain wall 108xe2x80x2. Thus, a readout magnetic domain 112 with an upward moment exists in the region 110 irradiated with the light beam 106.
As described above, the independent magnetic domain 109 is read out once when it is located at the front end of the region X where the second magnetic layer 102 is heated to its Curie temperature or above by the irradiation of the light beam (in the state shown in FIG. 11), and read out again when it is located at the rear end of the region X (in the state shown in FIG. 12). This phenomenon is noticeable in a relatively long recording magnetic domain where the exchange coupling between the third magnetic layer 103 and first magnetic layer 101 is stable as disclosed in xe2x80x9cHigh-Density Magneto-Optical Recording with Domain Wall Displacement Detectionxe2x80x9d (Joint Magneto-Optical Recording International Symposium/International Symposium on Optical Memory 1997 Technical Digest, Tu-E-04, p. 38,39).
Thus, with a conventional magneto-optical recording medium, since a relatively long recording magnetic domain can not be read out in a stable manner, a serious problem will occur when performing recording and reproduction by a mark edge recording method in which information is recorded at a higher density.
An object of the present invention is to provide a magneto-optical recording medium which enables readout of signals of a cycle equal to or less than a diffraction limit of light without lowering the amplitude of the readout signals and does not cause repetitious readout even in a long recording magnetic domain.
In order to achieve the above object, a magneto-optical recording medium of the present invention includes at least a first magnetic layer, a second magnetic layer, a third magnetic layer, a non-magnetic intermediate layer and a fourth magnetic layer which are layered in this order, and is characterized in that
the first magnetic layer is formed of a perpendicularly magnetized film with a relatively small wall coercivity and a relatively large wall mobility compared with the third magnetic layer in the vicinity of a predetermined temperature,
the second magnetic layer is formed of a magnetic film whose Curie temperature is lower than those of the first and third magnetic layers, and
the fourth magnetic layer is a perpendicularly magnetized film which has a uniform magnetization direction and forms a region with a uniform magnetization direction in the first magnetic layer by magnetostatic coupling with the first magnetic layer when the second magnetic layer is heated to a predetermined temperature or a higher temperature.
According to this structure, during reproduction, a region with a uniform magnetization direction can be formed in the first magnetic layer by magnetostatic coupling with the fourth magnetic layer, and a movement of the domain wall from the rear end of the light beam can be limited by the region with the uniform magnetization direction.
Therefore, even when a recording magnetic domain is long, it can be readout accurately. Thus, this structure can cope with high-density mark edge recording.
Consequently, magnetic domain enlargement readout is realized without causing repetitious readout, and signals of a cycle less than the diffraction limit of light can be read out without lowering the amplitude of the readout signals, thereby significantly improving the recording density.
Moreover, in order to reproduce information on the above-mentioned magneto-optical recording medium, a reproducing device of the present invention is characterized in including:
irradiating means for irradiating a light beam on the magneto-optical recording medium during reproduction; and
control means for controlling the irradiation intensity of the light beam to an intensity capable of heating the magneto-optical recording medium to a temperature at which magnetostatic coupling between the fourth magnetic layer and the first magnetic layer occurs, or a higher temperature.
In other words, the reproducing device of the present invention is designed to heat the magneto-optical recording medium to a predetermined temperature (readout temperature) or a higher temperature by the irradiation of the light beam by the irradiating means, and to control the light beam which is controlled to an intensity capable of generating a leakage magnetic field sufficient for achieving magnetostatic coupling between the fourth magnetic layer and the first magnetic layer to irradiate the magneto-optical recording medium by the control means. Thus, with the use of this reproducing device, the information on the magneto-optical recording medium including the fourth magnetic layer can be satisfactorily reproduced.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the companying drawings.